Promise 如何与嵌套函数调用一起工作

Question

我有一段处理用户数据的代码。有一堆嵌套的函数调用：

f1(){
   f2(){
     ....
      fn{
        ///
      }
   }
}

fn 访问一个数据库，这意味着它是异步的，所以我以某种方式写了它返回一个 promise 并在 fn-1（调用 fn 的函数）中，我们使用 .then() 等待这个承诺。但看起来现在我必须在fn-1 中返回一个承诺，依此类推。真的吗 ？

var keyValueExists = function(key, value) {
    var query = {};
    query[key] = value;
    return new Promise(function(resolve, reject) {
        User.count(query, function(err, count) {
            if (err) {
                console.log(err);
                console.log('Problem with `.find` function');
                reject('Problem with `.find` function');
            } else {
                resolve(count !== 0);
            }
        });
    });
};

var addUser = function(newUserInfo) {
    var validationResult = Common._validateUserInfo(newUserInfo);
    if (validationResult.isOK) {
        keyValueExists('userName', newUserInfo.userName).then(function(userNameAlreadyExists) {
            if (userNameAlreadyExists) {
                validationResult = {
                    isOK: false,
                    reason: 'Username already exists',
                    infoWithBadInput: 'userName'
                }
            } else {
                var newUserId = generateUserId();
                //TODO: change it somehting more flexible. e.g. a predefined list of attributes to iterate over
                var newUser = {
                    'userName': newUserInfo.userName,
                    'password': newUserInfo.password,
                    'userId': newUserId,
                    'lastModificationTime': Common.getCurrentFormanttedTime(),
                    'createdTime': Common.getCurrentFormanttedTime()
                };
                var user = new User(newUser);
                user.save(function(err) {
                    if (err) {
                        console.log(err);
                        console.log('There is a problem saving the user info');
                    } else {
                        console.log('A new user added: ');
                        console.log(newUser);
                    }
                });
            }
            return validationResult;
        });
    } else {
        return validationResult;
    }
};

addUser 返回undefined ！看起来addUser 的调用者没有等待它！

Answer 1

这就是您在 addUser 函数中有效执行的操作

var addUser = function(newUserInfo) {
    var validationResult = Common._validateUserInfo(newUserInfo);
    if (validationResult.isOK) {
        // ... do something asynchronously without returning anything
    } else {
        return validationResult;
    }
}

所以，是的，如果validationResult.isOK，adduser 将返回 undefined

这里有一些大致基于您的代码的代码，但它独立运行以演示您应该如何做事

var keyValueExists = function(key, value) {
    // pseudo junk, this simulates any username starting with b as existing
    return new Promise(function(resolve, reject) {
        resolve(value.substr(0,1) == 'b'); // barny and betty are dupes, fred and wilma are not
    });
}

var addUser = function (newUserInfo) {
    // var validationResult = Common._validateUserInfo(newUserInfo);
    var validationResult = {isOK: !!~(['fred', 'barny'].indexOf(newUserInfo.userName)), username: newUserInfo.userName}; // dummy code
    if (validationResult.isOK) {
        return keyValueExists('userName', newUserInfo.userName).then(function (userNameAlreadyExists) {
            if (userNameAlreadyExists) {
                validationResult = {
                    isOK: false,
                    reason: 'Username already exists',
                    infoWithBadInput: 'userName',
                    username: newUserInfo.userName
                }
            } else {
                // create new user here
                validationResult.userNumber = (Math.random() * 100000000) | 0;
            }
            return validationResult;
        });
    }
    else {
        // this function always needs to return a promise, even if it is resolved/rejected immediately
        return Promise.reject(validationResult);
    }
}

addUser({userName: 'fred'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});

addUser({userName: 'wilma'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});

addUser({userName: 'barny'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});

addUser({userName: 'betty'}).then(function (result) {
    console.log(result);
}).catch(function(err) {
    console.error(err);
});